Regarding an internal combustion engine mounted in an automobile or the like, a technique is known according to which a turbocharger for compressing intake air is provided in order to achieve an improvement in charging efficiency to thereby improve the engine output. Generally speaking, such a turbocharger is driven by utilizing the energy of exhaust gas discharged from the internal combustion engine.
In a turbocharger, a turbine housing provided at some midpoint in an exhaust passage and a compressor housing provided at some midpoint in an intake passage are connected to each other through the intermediation of a center housing, and a turbine wheel rotatably supported in the turbine housing and a compressor wheel rotatably supported in the compressor housing are coaxially connected through the intermediation of a turbine shaft rotatably supported in the center housing.
In such a turbocharger, exhaust gas discharged from the internal combustion engine flows into the turbine housing through an exhaust inlet, and this exhaust gas flows along a scroll passage in an eddy-like fashion. Then, it flows from the scroll passage to a nozzle passage before it is blown against the turbine wheel to thereby rotate the turbine wheel.
When the turbine wheel is thus rotated, the torque of the turbine wheel is transmitted to the compressor wheel through the turbine shaft, and the compressor wheel rotates in synchronism with the turbine wheel. When the compressor wheel rotates in synchronism with the turbine wheel, the intake air in the vicinity of the intake air inlet is sucked in the compressor housing by a sucking force generated by the rotation of the compressor wheel and sent under pressure to an intake air outlet by way of a send-out passage and the scroll passage.
Thus, the intake air compressed in the compressor housing is forcibly supplied to the combustion chamber, so that the charging efficiency of the intake air is improved. In this process, the fuel injection amount is increased in response to the increase in the intake air amount, whereby it is possible to obtain larger combustion power and explosive power, making it possible to enhance the engine output.
At this time, the turbine wheel must rotate at a high speed of from 100,000 to 160,000/min. while being exposed to exhaust with a maximum temperature as high as 900° C. Thus, in the production of a turbocharger, the turbine wheel, the compressor wheel, and the turbine shaft must be arranged with high accuracy in the same rotation axis. In particular, it is very important that no production error (deviation in rotation axis of the wheel and the turbine shaft) should be generated when joining them together.
Conventionally, the wheel and the turbine shaft are often joined by electron beam welding; in this case, the product accuracy depends on the accuracy with which the pre-welding processing (edge preparation) is performed.
Conventionally, this edge preparation has been performed as follows.
First, as shown in FIG. 9, a fitting hole 51 is formed in a turbine wheel 50, and a protrusion 61 is formed at one end of one turbine shaft 60 on the side joined to the turbine wheel 50. This protrusion 61 is fitted into the fitting hole 51 so as to generate a gap portion 52, and one end of the turbine shaft 60 is abutted against the turbine wheel 50 at an abutment portion 53 to perform positioning.
In another method, the turbine wheel and the turbine shaft are abutted against each other, and positioning is performed in a condition in which they are secured by a welding jig.
Of those conventional methods, the former method requires provision of a clearance at the fitting portion 52 taking into account the deformation at the time of welding, etc., so that, due to the play, it is rather difficult to secure the coaxiality of the turbine wheel 50 and the turbine shaft 60.
Further, at the time of joining, the entire periphery of the abutment portion 53 is fused by electron beam welding or the like, and the fusion of the abutment portion 53 is likely to lead to bending deformation at this portion.
Further, since the turbine shaft 60 is contracted in the axial direction, a problem occurs such as the dimensional accuracy in the axial direction is likely to be lost.
In the latter method, the positioning of the turbine wheel 50 and the turbine shaft 60 depends upon the accuracy of the jig used, so that it is rather difficult to secure stable coaxiality. Further, due to the variation in the jig and secular change, it is difficult to maintain accurate coaxiality.
In addition, as in the former method, the entire abutment portion of the turbine wheel and the turbine shaft is fused by electron beam welding or the like so that bending deformation is likely to occur at this portion. Further, since the turbine shaft contracts in the axial direction, a problem occurs such as the dimensional accuracy in the axial direction is likely to be lost.
In particular, in the above conventional methods, a part of the turbine shaft (abutment portion 53) is fused by welding, so that the turbine shaft 60 contracts. In view of this, the turbine wheel 50 and the turbine shaft 60 are first welded, and, thereafter, as shown in FIG. 10, adjustment of the bending of the shaft main body of the turbine shaft 60 and minute processing of the thrust bearing, etc. provided at one end thereof must be executed for improvement in general accuracy. Specifically, after welding the structure with a contour as indicated by the solid line in FIG. 10, this turbine shaft 60 has to be cut into the shape as indicated by the two-dot chain line, executing adjustment of the axis and minute processing of the thrust bearing, etc. Thus, as compared with the case in which processing is performed solely on the turbine shaft 60 before welding, the processing is hard to perform and requires a lot of time.
The present invention has been made in view of the above problems. It is a technical object of the present invention to provide a joining method which makes it possible to achieve an improvement in the joining accuracy for the wheel and the turbine shaft.